Method of making a sensor and the product produced therefrom

ABSTRACT

A conductive co-fired body for an electrochemical cell for an exhaust sensor comprises zirconia, yttrium oxide, and alumina. The body comprises about 15 to about 30 weight % monoclinic phase zirconia. This produces an electrochemical cell having low impedance wherein the zirconia body and alumina body are co-fired. One method for manufacturing the electrochemical cell comprises combining zirconia, yttria, and alumina with solvent and dispersant to form a mixture. After, binder is added to the mixture which is then de-aired and cast onto a tape surface. The tape is dried, metallized, and laminated to an unfired alumina surface. The structure is then co-fired to form a body for said electrochemical cell.

TECHNICAL FIELD

[0001] The invention relates generally to exhaust sensors for automotiveapplications. Particularly, the present invention relates to an exhaustsensor having a high conductivity co-fire zirconia body.

BACKGROUND OF THE INVENTION

[0002] The automotive industry has used exhaust sensors in automotivevehicles for many years to sense the oxygen content of exhaust gases.For example, sensors are used for alteration and optimization of the airto fuel ratio for combustion.

[0003] For oxygen sensing, solid electrolyte sensors are used to measureoxygen partial pressure differences between an unknown gas sample and aknown gas sample. A sensor typically has a conductive solid electrolytebetween porous electrodes. In the use of a sensor for automotiveexhaust, the unknown gas is exhaust and the known gas (i.e., referencegas) is usually atmospheric air because oxygen content in the air isrelatively constant and readily accessible. This type of sensor is basedon an electrochemical galvanic cell operating in a potentiometric modeto detect the relative amounts of oxygen present in an automobileengine's exhaust. When opposite surfaces of this galvanic cell areexposed to different oxygen partial pressures, an electromotive force(emf) is developed between the electrodes according to the Nerstequation:$E = {\left( \frac{R\quad T}{4F} \right){\ln \left( \frac{P_{O_{2}}^{ref}}{P_{O_{2}}} \right)}}$

[0004] where:

[0005] E=electromotive force

[0006] R=universal gas constant

[0007] F=Faraday constant

[0008] T=absolute temperature of the gas

[0009] P_(O) ₂ ^(ref)=oxygen partial pressure of the reference gas

[0010] P_(O) ₂ =oxygen partial pressure of the exhaust gas

[0011] For proper sensor operation, interfacial impedance of thesensor's electrochemical cell should be maintained within an effectivetemperature range which is dependent upon the cell's composition. Withprior art cell formulations, some would often have excessively highelectrode/electrolyte interfacial impedance after being kiln fired(firing). The cause of the poor impedance performance of the cells isrelated to the impurity content of the cell materials, cleanliness inmanufacture, and sintering temperature.

[0012] Low electrochemical cell impedance is achievable with a varietyof co-synthesized yttria stabilized zirconia bodies currently availableor described in literature. Some of these have demonstrated a high ionicconductivity, high microstructural homogeneity, and good low temperaturestability. However, these materials are incapable of being laminated orotherwise joined to an alumina body in the green (unfired) stagefollowed by a firing to a high density level (theoretical) of about 93%or higher. These materials typically fail by a crack or separation at orassociated with the alumina body/zirconia body interface because ofsintering shrinkage and/or thermal expansion mismatch in the firingmanufacturing process. Some prior art zirconia body formulations,however, are compatible with some high alumina body formulations. Thisis primarily because these zirconia body formulations convert about 20weight % to a monoclinic phase from a tetragonal phase while coolingfrom the firing process between about 650° C. and about 350° C. However,these formulations, as stated above, cannot be made into anelectrochemical cell with low, stable, electrode impedance.

SUMMARY OF THE INVENTION

[0013] The deficiencies of the above-discussed prior art are overcome oralleviated by the method of manufacturing a zirconia-alumina body, and asensor of the present invention. The method of manufacturing thezirconia-alumina body comprises: mixing zirconia, yttria, and aluminawith at least one solvent to form a mixture; drying said mixture;laminating said dried mixture to an unfired alumina surface; andco-firing to form the zirconia-alumina body, wherein saidzirconia-alumina body comprises about 1 weight % to about 45 weight %monoclinic phase zirconia, based upon the total weight of the zirconia.

[0014] The method of making the sensor comprises: mixing zirconia,yttria, and alumina with at least one solvent and at least onedispersant to form a mixture; drying said mixture to form an unfiredzirconia body; disposing an electrode on each side of said unfiredzirconia body; connecting each electrode to an electrical lead;disposing said unfired zirconia body adjacent to an unfired aluminasurface to form an unfired zirconia-alumina body, wherein one of saidelectrodes is disposed between said zirconia body and said alumina body;and co-firing to form the sensor, wherein the co-fired zirconia-aluminabody comprises about 1 weight % to about 45 weight % monoclinic phasezirconia, based upon the total weight of the zirconia.

[0015] The above-discussed and other features and advantages of thepresent invention will be appreciated and understood by those skilled inthe art from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will now be described, by way of exampleonly, with reference to the accompanying drawings, which are meant to beexemplary, not limiting, and wherein like elements are numbered alike inseveral Figures, in which:

[0017]FIG. 1 is a layout of a planar oxygen sensor in accordance withthe present invention;

[0018]FIG. 2 is a graphical illustration of normalized current densityshowing a variable current density over an applied voltage for freshsamples and the same samples aged in 800° C. in air;

[0019]FIG. 3 is a graphical illustration of a DC volume resistivityshowing a variable resistivity over an applied voltage for fresh samplesand the same samples aged in 800° C. in air;

[0020]FIG. 4 is a graphical illustration of a zirconia body electrodeimpedance stability comparison of variable electrode impedance over timein air at 800° C.;

[0021]FIG. 5 is a graphical illustration of thermal expansion andcontraction for an alumina body, cubic/tetragonal-zirconia body, and oneembodiment of a zirconia body of the present invention comprising about22 weight % monoclinic phase zirconia.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] The device explained herein is a sensor for sensing an unknowngas using a known reference gas. The formulations disclosed herein arerelated to the production of an oxygen sensor. However, it should beunderstood that other gases could be sensed depending on the unknown gasand known reference gas, and the composition and structure of theelectrode materials.

[0023] The sensor has a solid electrolyte, such as yttria doped zirconiaor the like, that is bound by at least two electrodes, and a series ofsubstrates such as alumina tapes. The zirconia comprises up to about 45weight percent (weight %) monoclinic phase, based upon the total weightof the zirconia, which enables compatibility with the alumina tapes. Inoperation, the first electrode is exposed to the sensing atmosphere suchas exhaust gas. The second electrode is exposed to a reference gas suchas oxygen. Usually, one or more heaters are attached to the device tokeep the device at a sufficient temperature for sensing operation. Theemf measured from the two electrodes, due to the galvanic potential,represents the partial pressure difference between the sensingatmosphere and the reference gas. For an automobile exhaust system, thisdifference is indicative of the oxygen content in the exhaust gas.

[0024] Referring to FIG. 1, an example of a planar oxygen sensor 17 isshown. For this arrangement, a solid electrolyte tape 1 is yttria dopedzirconia, tapes 2 are alumina, and tape 3 is an alumina-zirconia mixedtape which provides particulate protection. Under tape 3 is the exhaustgas sensing electrode 4 which connects to the contact pad 5 through thelead 6, while the reference electrode 7, which connects to the pad 8through lead 9, is disposed in fluid communication with referencechamber 10. Proximate alumina tapes 2, heater 11 is connected to contactpad 13, which is connected to corresponding leads 14 and 16. Theelectrodes, leads, contact pads, and heater can comprise materialsconventionally employed in the sensors, such as platinum, palladium,rhodium, osmium, iridium, ruthenium, and other metals, metal oxides, andother materials, as well as alloys and mixtures comprising at least oneof the foregoing. Furthermore, other conventional components may beemployed such as a lead gettering layer, ground plane, and the like.

[0025] A formulation for producing a conductive co-fired body, forexample for tape 1 above, comprises, based upon the total weight of theco-fired body, up to about 95 mole % zirconia (ZrO₂), with about 85 toabout 93 mole % preferred; up to about 10 mole % yttrium oxide (Y₂O₃),with about 3 to about 7 mole % preferred; and up to about 10 mole %alumina (Al₂O₃), with about 3 to about 7 mole % preferred; wherein afterprocessing and firing, about 1 weight % to about 45 weight % of thezirconia is monoclinic phase zirconia, with about 15 weight % to about30 weight % preferred, and about 18 weight % to about 25 weight %especially preferred, balance cubic and tetragonal phases . Preferably,a sufficient amount of the zirconia is in the monoclinic phase such thatthe sintering curve of the zirconia body tape and the alumina body tape(measured individually via a sintering dilatometer method), have asintering mismatch of about 5% or less. The yttrium oxide added hereacts as a stabilizer. The zirconium oxide is generally purer than thatused in prior art formulations, e.g., the zirconia comprises less thanabout 100 parts per million (ppm) of each of silica, sodium, calcium,magnesium, iron, titanium, chlorine, and other impurities, with a totalimpurity amount of less than about 1,000 ppm more preferred, and a totalof less than about 500 ppm more preferred, and a total of less thanabout 300 ppm especially preferred. By having minimal impurity levels,especially silicon (Si), in the zirconia body batch ingredients, theeffects of diffusion of impurity species to the electrode/electrolyteinterface are minimized. This helps attain low overall cell impedance;for example, an electrode resistivity about 10 ohm-cm or lower at 800°C. in air, and a total cell bulk DC resistivity about 250 ohm-cm orlower at 800° C. in air.

[0026] In theory, the zirconia body, which generally is in tetragonal,monoclinic, or cubic phase, is compatible with the alumina body becausethe fired zirconia body phase chemistry includes up to about 45 weight %of monoclinic, balance cubic and tetragonal phases. While the monoclinicphase content reduces the ionic conductivity of the electrochemicalcell, it provides critical structural compatibility with alumina bodies.This thereby enables the production of co-fired, monolithic, hybridizedzirconia body/alumina body structures. Furthermore, this formulationenables stress relief upon cooling of co-fired alumina/zirconiaelements, thereby inhibiting cracking or separation failures.

[0027] Referring to FIG. 5, line 50 represents alumina body contractionon cooling, line 52 shows a cubic/tetragonal zirconia body on cooling,and line 54 shows the cooling curve of a zirconia body comprising about22 weight % monoclinic phase. Note how, around 500° C., line 54 suddenlyrises, showing the volumetric expansion of the tetragonal to monoclinictransformation. The mismatch of line 52 (which is a formulation likeSample 1 below) becomes worse relative to the alumina, line 50, ascooling continues.

[0028] The process for producing a conductive co-fired body includesforming a batch mixture of zirconia, yttria, and alumina along withsolvent(s) such as xylenes, ethanol, and the like and/or dispersant(s)such as phosphate ester, Menhaden fish oil, sulfosuccinate, castor oil,and the like. This mixture is milled for a sufficient period of time toobtain a substantially homogeneous mixture, e.g., typically about 4 toabout 12 hours. Thereafter, binder(s) (such as polyvinyl butyral, polymethyl methacrylate, poly vinyl formol, and the like), andplasticizer(s) (such as butyl benzyl phthalate, glycols (e.g.,polyethylene glycol, and the like) and phthalates, (e.g., dimethylphthalate, octyl phthalate, and the like) and others), can optionally beadded to the mixture. The mixture is preferably mixed, e.g., milled, foran additional period of time to obtain a substantially homogeneousmixture, e.g., typically up to about 8 hours or so, to produce a slurry.The slurry produced is then preferably de-aired, which is typicallyachieved by pulling a vacuum on the slurry for up to about 3 minutes orso.

[0029] After de-airing, the slurry is formed into the tape by a knownmethod, e.g., it is cast using a known doctor blade tape casting method.Typically, the slurry will be cast onto a carrier such as polyesterfilm. Once the tape has been cast, the tape is allowed to dry therebyproducing unfired zirconia body tape. This tape may then be metallizedusing a thick film screen printing process, or other technique, todispose the electrodes on the tape (one on each side). The tape is thenlaminated to an unfired alumina body tape which typically has up toabout 96 weight % alumina body, with up to about 98 weight % preferred;and up to about 10 weight % flux, with about 4 weight % to about 6weight % flux preferred.

[0030] The flux may be a mineral based composition including clay, talc,calcium carbonate, and the like, and may be used in a “fritted” form(pre-reacted to form a glass, which is then ground.) A preferred flux isa frit containing silica, lanthanum oxide, yttrium oxide, boron oxide,alumina, and other metal oxides, as well as mixtures comprising at leastone of the foregoing.

[0031] The laminated structures are co-fired in an air atmosphere kilnat a sufficient temperature to achieve closed porosity of the denseceramics (typically about 1,375° C. to about 1,550° C., with atemperature of about 1,500° C. to about 1,530° C. preferred; with a holdfor up to about 2 hours or so). Once fired, the co-fired body isdisposed next to the remainder of the desired sensor components.

[0032] Alternatively, all components can be assembled prior to firing,and then co-fired in a single process to form an integral sensor. Inthis embodiment, for example, a protective layer may be disposedadjacent to one electrode while the alumina body is disposed adjacent tothe other electrode. Other layers which may also be employed include alead gettering layer disposed between the protective layer and theelectrode, and support layers disposed adjacent to the opposite side ofthe alumina body. Disposed within the support layers can be a groundplane and a heater. Necessary leads, contacts, and vias are also formedon the appropriate layers to connect the electrodes, ground plane andheater(s) accordingly, as is well known in the art.

[0033] Examples of formulations for the zirconia body are shown inTables I and II, wherein Table I shows the total slurry formulation andTable II shows the inorganic batch weight percentages. TABLE I TotalSlurry Formulation (g) Sample Number Ingredient 1 2 3 4 5 6 7 8 9 YttriaStabilized 285.5 214.1 142.8 71.4 0.0 214.1 142.8 71.4 0.0 Zirconia, 5mole % High Purity 0.0 65.7 131.3 197.0 262.7 0.0 0.0 0.0 0.0 MonoclinicZirconia¹ Monoclinic Zirconia 0.0 0.0 0.0 0.0 0.0 65.7 131.3 197.0 262.7High Purity Yttria 3.0 8.7 14.4 20.1 25.8 8.7 14.4 20.1 25.8 Alumina11.5 11.5 11.5 11.5 11.5 11.5 11.5 11.5 11.5 Xylenes (solvent) 56.4 56.456.4 56.4 56.4 56.4 56.4 56.4 56.4 Ethanol (solvent) 56.4 56.4 56.4 56.456.4 56.4 56.4 56.4 56.4 Phosphate Ester 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.04.0 Dispersant Butyl Benzyl Phthalat 13.2 13.2 13.2 13.2 13.2 13.2 13.213.2 13.2 Plasticizer Polyvinyl Butyral 22.0 22.0 22.0 22.0 22.0 22.022.0 22.0 22.0 Binder

[0034] TABLE II Weight Percent Oxide Sample Number Ingredient 1 2 3 4 56 7 8 9 Yttria Stabilized 95.2 71.4 47.6 23.8 0.0 71.4 47.6 23.8 0.0Zirconia, 5 mole % High Purity 0.0 21.9 43.8 65.7 87.6 0.0 0.0 0.0 0.0Monoclinic Zirconia Monoclinic 0.0 0.0 0.0 0.0 0.0 21.9 43.8 65.7 87.6Zirconia High Purity Yttria 1.0 2.9 4.8 6.7 8.6 2.9 4.8 6.7 8.6 Alumina3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 Total 100.0 100.0 100.0 100.0 100.0100.0 100.0 100.0 100.0

[0035] Formulation Samples 2 through 5 disclose preferred embodiments,with Sample 5 being most preferred. Sample 9 represents a standardoxygen sensor formulation. FIGS. 2 and 3 show comparisons of currentdensity and DC volume resistivity, respectively, for Samples 1, 5, 6,and 9. The average DC current and resistance of the Samples weremeasured by applying 0.1 volts (V) to 1.1V, at 0.1V increments, withabout one minute dwell at each voltage. A precision resistor was used inseries for a voltage measurement. Current density and resistivity werecalculated with the cell thickness and electrode area. The Samples weremeasured in air at 800° C. initially, and after air aging at 800° C. for24 and 48 hours.

[0036] Sample 1, lines 21, 22, and 23, demonstrates a good currentdensity and resistivity with good retention of these properties overtime. This Sample, however, has no detectable level of monoclinic phasezirconia present after firing. Consequently, this Sample 1 would beplagued with the conventional problems of thermal mismatch, resulting incracking.

[0037] Sample 5, which also demonstrates good current density andresistivity, when fired at about 1510° C. for 2 hours, contains about 15weight % to 30 weight % monoclinic phase, as measured at roomtemperature by x-ray diffraction. The balance of the phase chemistry iscubic and tetragonal. With this phase chemistry, Sample 5 can beco-fired with alumina body to make a monolithic, hybrid structure. Yet,as can be seen in FIG. 2, the current density with applied voltage isquite linear, and the linearity and magnitude is the most stable overtime of the illustrated Samples (lines 24-26). One possible explanationfor this behavior is the relative purity of the electrolytes; Sample 5contains a high purity monoclinic zirconia powder that has less than 100parts per million (ppm) silicon. Samples 6 through 9 contain betweenabout 500 and 1,000 ppm silicon, with a relative conductivity of thezirconia phases being more tetragonal and cubic than monoclinic. Theresistivity with applied voltage in FIG. 3 shows how aging andelectrical energy imposed on the cell has an effect on the resistivitylevel. Sample 9 has a relatively flat plot initially (line 31), butafter 24 (line 32) and 48 hours (line 33), the resistivity at lowapplied voltages is several times higher. As voltage is applied to theseair aged samples, the resistivity drops closer to its initial value,around 1 applied volt. Samples 1 and 6 also show this behavior to alesser degree (lines 34-36 and 37-39, respectively). Sample 5 isrelatively flat and stable at 0 (line 40), 24 (line 41), and 48 hours(line 42).

[0038] A further illustration of the cell stability in air at 800° C. isshown in FIG. 4. FIG. 4 shows an illustration of a zirconia bodyelectrode impedance stability comparison of electrochemical cells madefrom Samples 5 and 9, which were tested for over 450 hours. As can beseen, there is approximately three orders of magnitude difference in theelectrode impedance. Also, Sample 5 maintains its reduced electrodeimpedance over time (lines 42 and 43).

[0039] Firing the cell at lower sintering temperature also furtherlowers the electrode impedance and enhances stability. For example, inFIG. 4, Sample 5 shows about a 10% reduction in electrode resistivitywhen fired at 1,450° C. rather than 1,510° C. This reduction ismaintained over a 450 hour air aging time interval at 800° C.

[0040] Alternative embodiments are also possible. These includeformulations having the addition of co-synthesized yttria stabilizedzirconia powder while still retaining up to about 10 mole %, preferablyabout 5 mole %, of the total yttria content. The co-synthesized yttriastabilized zirconia increases the percent by weight of tetragonal andcubic phase in the fired zirconia body. This may increase the pumpingcurrent of the cell and effect low electrode and total cell impedance atlower temperatures. However, utilizing an excessive amount of theco-synthesized yttria stabilized material in the batch formulation(e.g., greater than about 75 weight % of the total zirconia) will resultin a fired body with little or no monoclinic phase. Formulations withmore than about 75 weight % of total zirconia added as co-synthesizedmaterial demonstrate weak volumetric expansion upon cooling; during thetetragonal to monoclinic polymorphic phase transformation that typicallyoccurs as the sintered body cools from about 650° C. to about 350° C.

[0041] The disclosed invention provides several improvements. First, upto about 45 weight %, preferably about 15 weight % to about 30 weight %,of the zirconia body after firing is in a monoclinic phase, whereinfully stabilized or tetragonal zirconia bodies have no monoclinic phasedetectable by x-ray diffraction. This enables co-firing with a highalumina body which thereby enables production of co-fired, monolithic,hybridized zirconia/alumina body structures. Second, the cell afterfiring has low electrode impedance (e.g. below about 10 ohm-cm) andtotal bulk DC resistivity (e.g., below about 250 ohm-cm). This shortensthe time to activity, reduces power consumption, and enables enhancedsensor performance (for example, enhanced range and/or sensitivity).Also, the low impedance cell has an improved stability and performancedue to the purer materials utilized.

[0042] While preferred embodiments have been shown and described,various modifications and substitutions may be made thereto withoutdeparting from the spirit and scope of the invention. Accordingly, it isto be understood that the present invention has been described by way ofillustrations and not limitation.

What is claimed is:
 1. A method of manufacturing zirconia-alumina body,comprising: mixing zirconia, yttria, and alumina with at least onesolvent to form a mixture; drying said mixture; disposing said driedmixture adjacent to an unfired alumina body; and co-firing to form thezirconia-alumina body, wherein said zirconia-alumina body comprisesabout 1 weight % to about 45 weight % monoclinic phase zirconia, basedupon the total weight of the zirconia.
 2. The method of manufacturingzirconia-alumina body of claim 1, further comprising mixing at least onedispersant into the mixture, and wherein the zirconia-alumina bodycomprises about 15 weight % to about 30 weight % monoclinic phasezirconia.
 3. The method of manufacturing zirconia-alumina body of claim2, wherein said dispersant is selected from the group consisting ofphosphate ester, Menhaden fish oil, sulfosuccinate, castor oil, andmixtures comprising at least one of the foregoing.
 4. The method ofmanufacturing zirconia-alumina body of claim 1, further comprisingadding at least one binder and at least one plasticizer to said mixture.5. The method of manufacturing zirconia-alumina body of claim 4, furthercomprises de-airing said mixture.
 6. The method of manufacturingzirconia-alumina body of claim 1, wherein the zirconia-alumina bodycomprises about 18 weight % to about 25 weight % monoclinic phasezirconia.
 7. The method of manufacturing zirconia-alumina body of claim4, wherein said at least one binder is selected from the groupconsisting of polyvinyl butyral, poly methyl methacrylate, poly vinylformal, and mixtures comprising of at least one of the foregoing.
 8. Themethod of manufacturing zirconia-alumina body claim 4, wherein said atleast one plasticizer is selected from the group consisting of butylbenzyl phthalate, glycols, phthalates, and mixtures comprising at leastone of the foregoing.
 9. The method of manufacturing zirconia-aluminabody of claim 1, wherein said laminated mixture and said alumina surfacehave a sintering mismatch of less than about 5%.
 10. The method ofmanufacturing zirconia-alumina body of claim 1, wherein said co-firingis performed at a temperature about 1,375° C. to about 1,550° C.
 11. Themethod of manufacturing zirconia-alumina body of claim 10, wherein saidco-firing is performed at a temperature of about 1,500° C. to about1,530° C.
 12. The method of manufacturing zirconia-alumina body of claim1, wherein said at least one solvent is selected from the groupconsisting of xylene, ethanol, and mixtures comprising at least one ofthe foregoing.
 13. The method of manufacturing zirconia-alumina body ofclaim 1, wherein the zirconia-alumina body comprises up to about 95 mole% zirconia, up to about 10 mole % yttrium oxide, and up to about 10 mole% alumina, based upon the total weight of the zirconia-alumina body. 14.The method of manufacturing zirconia-alumina body of claim 13, whereinthe zirconia-alumina body comprises about 85 to about 93 mole %zirconia, about 3 to about 7 mole % yttrium oxide, and about 3 to about7 mole % alumina, based upon the total weight of the zirconia-aluminabody.
 15. The method of manufacturing zirconia-alumina body of claim 1,further comprising metallizing the unfired zirconia body to form anelectrode on a first side and a second side of said zirconia body.
 16. Amethod of manufacturing a sensor, comprising: mixing zirconia, yttria,and alumina with at least one solvent to form a mixture; drying saidmixture to form an unfired zirconia body; disposing an electrode on eachside of said unfired zirconia body; connecting each electrode to anelectrical lead; disposing said unfired zirconia body adjacent to anunfired alumina surface to form an unfired zirconia-alumina body,wherein one of said electrodes is disposed between said zirconia bodyand said alumina body; and co-firing to form the sensor, wherein theco-fired zirconia-alumina body comprises about 1 weight % to about 45weight % monoclinic phase zirconia, based upon the total weight of thezirconia.
 17. A method of manufacturing a sensor as in claim 16, furthercomprising disposing a protective layer adjacent to said unfiredzirconia body on a side opposite said unfired alumina body.
 18. A methodof manufacturing a sensor as in claim 16, further comprising disposingsupport layers adjacent to said unfired alumina body, with a heaterdisposed within said support layers.
 19. A method of manufacturing asensor as in claim 18, further comprising disposing a ground plane insaid support layers, between said heater and said alumina body.
 20. Themethod of manufacturing zirconia-alumina body of claim 16, wherein thezirconia-alumina body comprises about 15 weight % to about 30 weight %monoclinic phase zirconia.
 21. The method of manufacturingzirconia-alumina body of claim 16, wherein the zirconia-alumina bodycomprises about 18 weight % to about 25 weight % monoclinic phasezirconia.
 22. The method of manufacturing zirconia-alumina body of claim16, further comprising adding at least one binder and at least oneplasticizer to said mixture.
 23. The method of manufacturingzirconia-alumina body of claim 22, further comprises de-airing saidmixture.
 24. The method of manufacturing zirconia-alumina body of claim22, further comprising at least one dispersant selected from the groupconsisting of phosphate ester, Menhaden fish oil, sulfosuccinate, castoroil, and mixtures comprising at least one of the foregoing.
 25. Themethod of manufacturing zirconia-alumina body of claim 22, wherein saidat least one binder is selected from the group consisting of polyvinylbutyral, poly methyl methacrylate, poly vinyl formal, and mixturescomprising of at least one of the foregoing.
 26. The method ofmanufacturing zirconia-alumina body claim 22, wherein said at least oneplasticizer is selected from the group consisting of butyl benzylphthalate, glycols, phthalates, and mixtures comprising at least one ofthe foregoing.
 27. The method of manufacturing zirconia-alumina body ofclaim 16, wherein said laminated mixture and said alumina surface have asintering mismatch of less than about 5%.
 28. The method ofmanufacturing zirconia-alumina body of claim 16, wherein said co-firingis performed at a temperature about 1,375° C. to about 1,550° C.
 29. Themethod of manufacturing zirconia-alumina body of claim 28, wherein saidco-firing is performed at a temperature of about 1,500° C. to about1,530° C.
 30. The method of manufacturing zirconia-alumina body of claim16, wherein said at least one solvent is selected from the groupconsisting of xylene, ethanol, and mixtures comprising at least one ofthe foregoing.
 31. The method of manufacturing zirconia-alumina body ofclaim 16, wherein the zirconia-alumina body comprises up to about 95mole % zirconia, up to about 10 mole % yttrium oxide, and up to about 10mole % alumina, based upon the total weight of the zirconia-aluminabody.
 32. The method of manufacturing zirconia-alumina body of claim 31,wherein the zirconia-alumina body comprises about 85 mole % to about 93mole % zirconia, about 3 mole % to about 7 mole % yttrium oxide, andabout 3 mole % to about 7 mole % alumina, based upon the total weight ofthe zirconia-alumina body.
 33. The sensor of claim
 16. 34. A method forsensing oxygen, comprising: disposing a sensor in an exhaust stream,said sensor comprising a co-fired zirconia body and an alumina body withan electrode disposed on a first side of said zirconia body and a secondelectrode disposed on a side opposite said first electrode, between saidzirconia body and said alumina body, wherein the co-fired comprisesabout 1 weight % to about 45 weight % monoclinic phase zirconia, basedupon the total weight of the zirconia; contacting said first electrodewith exhaust gas; and sensing oxygen in the exhaust gas.
 35. The methodfor sensing oxygen of claim 34, wherein the zirconia-alumina bodycomprises about 15 weight % to about 30 weight % monoclinic phasezirconia.